The invention relates to a turbo-charger for compressing the air to be supplied to an internal combustion engine with the use of a compressor which is driven by a turbine turned by the exhaust gases from the internal combustion engine, and, more particularly, to an exhaust bypass type turbo-charger, in which a portion of the exhaust gases to flow into the turbine is discharged by opening an exhaust bypass valve so that the rotating conditions of the turbine may be controlled.
For the demand for reduction in size and increase in efficiency of an internal combustion engine, wide use has been made recently of a turbo-charger for compressing the supply air to the engine with the use of the exhaust gases from engine. The compressor discharge pressure of the turbo-charger has to be maintained always at a preset pressure irrespective of the running condition of the engine. In order to satisfy this requirement, in the conventional turbo-charger, the discharge pressure of the compressor is partially discharged to the atmosphere in accordance with a known system so that the pressure downstream of the discharging portion may be maintained substantially at a constant level. It is, however, found that the discharge of the pressure which is raised by having made the compressor work invites high loss. With this in mind, there has been developed the so-called exhaust bypass type turbo-charger, i.e., a turbo-charger, in which a portion of the exhaust gases are discharged to reduce the loss before the exhaust gases work or drive the turbine of the turbo-charger.
Now, in this type of the exhaust bypass type turbo-charger, for example, shown in U.S. Pat. No. 4,211,811 entitled, Exhaust Bypassing System For A Turbocharger, the drive mechanism for opening and closing the exhaust bypass valve is controlled directly by the discharge pressure of the compressor. In this instance, it is desired that the compressor discharge pressure required of the exhaust bypass type turbo-charger has the characteristics that they are maintained as constant as possible from the time the bypass valve begins to be opened to the time the same is opened to its full stroke. However, the direct control of the drive mechanism by the compressor discharge pressure cannot effect, on principle, the control required at the constant compressor discharge pressure but can effect the control the compressor discharge pressure in proportion to the running condition of the engine because the drive mechanism is operated in proportion of the change in the compressor discharge pressure. As a result, the maximum of the compressor discharge pressure that is allowable from the standpoint of the mechanical strength of the engine is determined (generally at about 650 mm Hg in gauge pressure) so that the bypass valve opened, on the other hand, in order that the difference between the inside and outside of the bellows or the like of the drive mechanism may be such a pressure difference (generally at 200 mm Hg) as can ensure sufficient operation of the drive mechanism, the compressor discharge pressure at the beginning of the opening operation of the bypass valve has to be considerably reduced to a lower level than the pressure when the bypass valve is fully opened. As a result, there arises a disadvantage that the compressor discharge pressure at the beginning of the bypass valve opening operation becomes lower than the level required so that the drivability of the engine is accordingly deteriorated. As described hereinabove, since the working pressure of the drive mechanism has to be set as low as at 200 mm Hg, the spring constant of the drive mechanism has to be as low as possible. As a result, the drive mechanism is liable to become so instable that it is vibrated by the vibrations of a vehicle thereby to have its operations instabilized, and the drive mechanism has its intrinsic vibration frequency because of the minimized spring constant so that it becomes liable to be vibrated in response to the vibrations of the vehicle. In either event, another disadvantage resides in fact that the drive mechanism has its operating characteristics remarkably handicapped.